The present invention relates to wafer defect detection systems which use a scanning laser beam to illuminate a wafer under analysis and identify defects by analysis of reflected light or transmitted light. In particular, the present invention concerns a scanner system using multiple beams that concurrently illuminate a sample such as a wafer, a reticle, a mask and the like, under inspection and generate multiple corresponding reflected or transmitted beams that are concurrently detected.
A variety of systems are used for automated inspection of semiconductor wafers, in order to detect defects, particles and/or patterns on the wafer surface as part of a quality assurance process in semiconductor manufacturing processes. It is a goal of current inspection systems to have high resolution and high contrast imaging in order to provide the reliability and accuracy demanded in sub-micron semiconductor manufacturing processes. However, it is also important to have a high-speed process that permits a large volume throughput so that the quality and assurance processes do not become a bottleneck in the wafer production process. Accordingly, the optical inspection systems must use shorter wave lengths, higher numerical aperture optics and high density image capture technology in order to enable the processing of data from such systems at sufficiently high rates that will satisfy the desired product throughput requirements.
A conventional imaging architecture that is used in wafer inspection systems at this time utilizes a single spot scanning laser for high-speed imaging. However, the data rates achievable by such architectures are limited by the physical constraints that arise due to limitations in the speed and quality of the single laser beam, the applicable optical system and related detection devices. For example, the single laser acting as a point light source is focused as a spot onto the object under inspection and is scanned across the surface of the object, which may be stationary or moved on a stage mechanism in coordination with the scan. The reflected light from the object is then imaged onto a detector, which generates pixel data from the scanning process. The detector may be a CCD array, whose individual elements are positioned to receive the reflected light as the beam is scanned and be read our serially, in a conventional fashion. While a high resolution may be obtained from such point source illumination, the requirement to scan each point in the field in order to construct a viewable image subjects the system to a limitation on its throughput.
The scanning of the single laser beam may be accomplished by a rotating mirror system, as seen in U.S. Pat. No. 5,065,008 or an acousto-optic cell. However, these single spot scanning architecture necessarily have a limited speed and are possibly subject to scan aberrations, low illumination brightness and potential thermal damage to the specimen when high brightness laser sources are used. The high data rates required to inspect the submicron structures of current semiconductor products cannot be achieved, even when a stage-type scanning system is used that moves the specimen relative to a fixed illumination and image location while a synchronized scanning pattern is produced by moving the single point of light over an area at the fixed location.
Accordingly, there is a need for a specimen scanning system that will improve specimen throughput, while maintaining or even improving the reliability and accuracy of the data collected during the scan of a specimen, whether in a stationary or stage-type system. This need is satisfied by the present invention, by utilizing a plurality of parallel scanning beams to scan a specimen and by detecting a plurality of parallel reflected beams or parallel transmitted beams, depending on whether the specimen is to be inspected by reflecting light from a surface or by passing light through a surface, and processing the plural reflected or transmitted beams concurrently, such that the throughput is significantly enhanced over the single spot scanned system.
The present invention involves a system for inspecting a specimen using a single light source that provides a beam of light. The beam of light is imaged onto a traveling lens acousto-optic device that has an active region and is responsive to an RF input signal to selectively generate plural traveling lenses in the active region. The traveling lens acousto-optic device is operative to receive the light beam and generate plural spot beams, at the respective focus of each of the generated traveling lenses. The system also includes a light detector unit, comprising a plurality of detector sections, each detector section having a plurality of light detectors and at least one multi-stage storage device operative to receive in parallel an input from the plurality of light detectors. The information stored in each of the multi-stage storage devices is serially read out concurrently from the multiple stages.
In accordance with another feature of the present invention, the invention involves a system for inspecting a specimen comprising a source of a plurality of scanning spot beams. The beams are imaged on a surface of the specimen and a plurality of beams are produced therefrom by reflection from or transmission through the specimen. A light detector unit, having a plurality of detector sections, each detector section having a plurality of light detectors and at least one multi-stage storage device operative to receive in parallel an input from the plurality of light detectors, is used. The information stored in each of the multi-stage storage devices is to serially read out concurrently from the multiple stages.
In accordance with yet another feature of the present invention, the invention involves a method for inspecting a specimen. The method includes providing a plurality of flying spot beams from a single source of light and scanning the plurality of spot beams on a surface of a specimen, whereby a corresponding plurality of beams are generated by reflection from or transmission through the specimen. Then, the content of each of the reflected beams is captured and stored simultaneously in a respective signal storage section. The stored information is serially read, thereby improving speed and throughput of scanned data.
A further feature of the present invention is the achievement of an acousto-optic device that is adapted to receive a source of light and an RF input for generating a plurality of traveling lenses that provide a plurality of concurrent scanned spots. The device comprises a crystal medium having an active region, the active region defining a direction of acoustic wave propagation, an RF input portion for receiving a RF chirped input and being disposed at one end of the medium in a direction of acoustic wave propagation and a light input portion for receiving light from the source of light, the light input portion being disposed transverse to the direction of acoustic wave propagation, and a light output portion. A series of RF pulses input to the RF input portion is operative to generate a sequence of traveling lenses concurrently existing in the active region, the traveling lenses being operative to receive the light input to the light input portion and generate plural spot beams output from the light output portion.
Another feature of the present invention is a method of operating an acousto-optic device having an active region and adapted to receive a source of light at a light input portion and an RF input portion for generating a plurality of traveling lenses that provide a plurality of concurrent spots from a light output portion. The method comprises inputting to the RF input portion a series of chirped input pulses whereby an acoustic wave is formed for each input pulse and propagates in the active region in a propagation direction, inputting to the light input portion light from the source and in a direction transverse to said propagation direction, and applying the light to the propagating acoustic waves, the waves forming traveling lenses and concurrently existing in the active region, each of the traveling lens being operative to focus and direct said light, whereby a respective spot beam for each lens is output from the light output portion.
Yet another feature of the present invention is a linear light detector unit for detecting a plurality of concurrently scanning spot beams. The light detector unit comprises a plurality of adjacent light detector sections disposed linearly along a common axis. Each of the detector sections has a plurality of adjacent light detectors, and at least one multi-stage storage device operative to receive in parallel an input from the plurality of light detectors and to serially read out information stored in the multiple stages.
A further feature of the present invention is a method for detecting a plurality of pixels stored in a linear CCD having a first plurality of sections, each section comprising a second plurality of pixel storage elements and receiving an input from a respective one of a third plurality of concurrently scanning beams. The method comprises capturing and storing the content of each of the third plurality of beams simultaneously in a respective signal storage section and concurrently serially reading out the stored signals.
A yet further feature of the present invention is an apparatus for detecting both bright field and dark field images from the surface of a specimen, where the bright field image is detected by at least one CCD and the dark field image is detected by at least one high sensitivity detector, such as a photomultiplier tube (PMT).